Our overall long term goal in this laboratory is to understand the pathogenesis of the human autoimmune disease, membranous glomerulonephritis. We have been examining the only acceptable animal model for the disease, Heymann nephritis in rats. In particular we have focused on understanding the function of the putative autoantigen of the disease, gp33O. Just how an immune response against this rather large glycoprotein causes the observed dysfunction in Heymann nephritis leading to proteinuria is still in question. Recently, we have shown that in vitro plasminogen binds to gp33O and can be activated to plasmin by urokinase. Also, autoantibody obtained from the glomerular immune deposits of rats with active Heymann nephritis significantly inhibits the activation of plasminogen to plasmin by urokinase. We have also identified a 45-kDa kidney protein of unknown function that binds to a different site on gp33O. Our specific aims for this proposal are: l. to analyze the plasminogen system as it applies to rat glomerular epithelial cells under culture conditions. We will examine parameters such as plasminogen binding, activation kinetics, binding sites, possible internalization, inhibition and effect of nephritogenic autoantibody on the normal function of the plasminogen system. 2. to determine if plasminogen and the 45-kDa protein co-localize with gp33O in the immune deposits and if the relative quantity of gp33O or the 45-kDa protein in the kidney changes during the development of active Heymann nephritis. We will analyze these proteins by immunochemical microscopy and quantify mRNA levels by RT-PCR. Our hypothesis is that in vivo the glomerular epithelial cell has a stable cell-surface associated proteolytic potential in which gp33O serves as a receptor site for plasminogen which not only increases the activation rate of plasminogen by urokinase but also protects plasmin from inhibition by alpha2-antiplasmin. During active Heymann nephritis nephritogenic autoantibody affects gp33O and plasminogen on the surface of the glomerular epithelial cell causing immune complexes to form, inhibiting the activation of plasminogen bound to gp33O and causing a change in the synthesis of gp33O. These combined effects could lead to altered proteolytic activity of plasmin as well as other proteases and could contribute to the changes observed in the glomerular basement membrane permeability and epithelial cell morphology during active Heymann nephritis.